Depending on how you interpret the situation, the answer is either "yes" or "no." The Government Accountability Office (GAO) says the USPS is bound by law to keep delivering on Saturdays. But that's not how everyone sees it, as Reuters reports:

Republican Senator Tom Coburn of Oklahoma and Representative Darrell Issa of California on Thursday told the USPS Board of Governors to move forward with implementing the five-day delivery plan for mail.

"The Board of Governors has a fiduciary responsibility to utilize its legal authority to implement modified 6-day mail delivery as recently proposed," the lawmakers said in their letter to the USPS board.

The service is bound by law to deliver mail six days a week, and is incorrect in interpreting that a temporary measure used to fund U.S. government operations released it from that requirement, the GAO said in a letter to Representative Gerald Connolly, a Virginia Democrat, who requested that the watchdog agency look at the matter.

The plan to cut delivery of letter mail while retaining package delivery on Saturdays "rests upon a faulty USPS premise," GAO General Counsel Susan Poling said in the letter.

"Beginning the week of Aug. 5 this year," Donahoe said, USPS will provide "six days of package delivery and five days of mail delivery. ... We will not deliver or collect mail on Saturdays."

Its decision could, however, run into challenges from Congress and from unions that represent the Postal Service's employees. Donahoe made the case, though, that USPS has no choice. The Postal Service, which lost nearly $16 billion last year, will save about $2 billion a year with this change, Donahoe said.

Eliminating Saturday mail delivery, said Donahoe, is "just one part of a much larger strategy to return the Postal Service to long-term financial security." That strategy has included the closing of many facilities.

While both the economy and the rise of the Internet have had an impact on mail volumes, legislation from Congress itself has also put a real strain on USPS finances, as we reported in 2010:

It [Congress] ordered the U.S. Postal Service to prepay its future retirees' pension and health benefits. That's a cost of more than $5 billion a year.

Sen. Thomas Carper (D-DE), who chairs a Senate subcommittee on the post office, says the pension prepayments are a major cause of the Postal Service's red ink.

"We shouldn't ask the Postal Service what we ask of no other state and local government and, as far as I know, no other business enterprise to do — and that is to upfront set aside enormous amounts of money to meet health care needs of potential pensioners," he adds.